The probe found police showed “exceptional valor” in responding to the attack.

Prosecutors "typically review deadly police-involved shootings," a justice department official said, and there was no allegation of wrongdoing in the takedown of Aaron Alexis in September. For the investigation to last 11 months is not unusual, the official said.

Dramatic Photos: DC Navy Yard Shooting

Alexis, a 34-year-old contractor, killed 12 people in his rampage through the headquarters of Naval Sea Systems Command.

Alexis fired at police and police shot back in defense, according to the federal report.